Micro-Electro-Mechanical Systems “MEMS” devices typically have moving parts, and may require some form of anti-stiction feature to limit the ability of the moving part to stick to the substrate once it is free to move. One method to do this is to create small features that stick up from the substrate and limit the ability of the moving part to contact the substrate to only these areas. This can greatly reduce the forces that would cause the moving part to stick, eg. when the device is being dried after immersion in liquid during fabrication. One example of these anti-stiction features are metal pads or “bumps” used in creating a MEMS Gyro. These bumps are typically placed on top of another metal feature that is used for device interconnect and capacitive read out. These bumps can also be used to enhance electrical contact to the moving parts.
One method to create the metal features is to use image reversal photolithography, metal deposition and lift-off. This sequence can be repeated as many times as is required to create the desired structures. When using image reversal photolithography the patterned photoresist layer created by this method cannot be altered by using a second exposure in subsequent steps. Image reversal is desirable because it can create a profile in the resist pattern that aids in lift-off. The image reversal steps are much longer than a standard photolithography step, so an alternative process to reduce cycle time would also be desirable.
FIG. 2 shows a cross section of a current MEMS device integrated circuit. The device 22 consists of a substrate 28, a first metal 26 and an applied bump metal 24. In this example photoresist is applied to the substrate 28, then exposed and developed. The first metal 26 is then deposited and lifted off creating a substrate 28 with a first metal 26. The bumps 24 are added by applying a photoresist, exposing and developing the region of the bumps. The metal is then deposited and lifted off to create the bumps. There are two distinct metallization steps resulting in a first layer of metal 26 and a second layer of the metal, the bump metal 24.